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Science to Practice |
Department of Radiology, University of Iowa College of Medicine, 200 Hawkins Dr, Iowa City, IA 52242-1077 e-mail: brian-mullan@uiowa.edu
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Acute lung injury in acute respiratory distress syndrome, or ARDS, affects more than a million people worldwide each year. It is difficult to treat, and—despite best efforts—carries a mortality rate of 60% (1). Placing patients with acute lung injury in the prone position has been shown to improve oxygenation (2). In this issue of Radiology, Lee et al (3) provide insight into the physiology underlying this improvement by reporting the in vivo computed tomographic (CT) evaluation of ventilation and perfusion gradients in prone dogs with acute lung injury.
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Although patients with ARDS are hypoxemic, most die of multiple organ failure (4). This is thought to be due to cytokines released from lungs that are damaged by repeated cycles of overinflation and collapse brought on by mechanical ventilation. Since the mid-1970s, intensivists have found that approximately 70% of patients with acute lung injury will have improved oxygenation in the prone position (2). This is thought to be caused by the equalization of the dorsal-ventral ventilation and perfusion gradients in the lung. In the supine position, there is relatively more blood present dorsally and more aeration present ventrally because of gravity. Placing an animal prone does not reverse this gradient but rather equalizes it along the dorsal-ventral axis. Several factors that account for this include the effects of gravity and the normal pulmonary vascular architecture that favors flow to the dorsal portion of the lung. This equalization of gradients may be responsible for the observation that the prone position decreases ventilator-induced lung injury. Additional mechanisms may play a role, including a cephalocaudal redistribution of lung inflation and perfusion and changes in the shape of the diaphragm and the chest wall. Lee et al investigated the cephalocaudal distribution of inflation and perfusion by using a standard dog model of acute lung injury. They confirmed previous observations regarding the ventral-dorsal axis and demonstrated an equalization of a cephalocaudal gradient, as well. They showed that this equalization was due to an expansion of alveoli in the caudal region and an alveolar contraction in the cephalic region. Thus, the net alveolar volume of the lung was maintained.
The Practice
Clinical use.—In acute lung injury, a maneuver such as prone positioning can have a profound effect. There are potential complications with the prone position, however, and it is not easy to place a patient in the prone position when they are in the intensive care unit. Moreover, prone positioning does not result in improvement for all patients. In one study, Gattinoni et al (5) found no decrease in mortality. This may be due to the fact that the patients were placed prone for only 7 hours each day (and thus spent most of their time in the supine position) and that the use of the prone position was limited to 10 days. However, a post hoc analysis showed a decrease in mortality among the most severely ill patients. Identification and quantification of ventilation and perfusion gradients such as those described by Lee et al may guide the intensivist in decisions such as which patients would benefit from prone positioning and for how long.
Further, this work highlights a new role for radiologists. Traditionally, radiologists have imaged a patient’s anatomy. The current generation of CT scanners and magnetic resonance imagers opens the window of direct physiologic imaging. Won et al (6) demonstrated the ability to quantify capillary blood flow from CT data. Tajik et al (7) showed the ability to measure subsecond regional pulmonary ventilation. Lee et al (3) show how such techniques can be used to understand empiric observations. Combining such sensitive measurements of physiologic function allows insight into the evaluation and clinical management of lung disease.
Future opportunities and challenges.—Recent advances in clinical CT technology have provided a new opportunity for patient care–namely, physiologic imaging. Although physiologic imaging has been well explored in the laboratory, transitional research is needed to bring what is being done in the laboratory (the science) into the clinic (the practice). Radiologists who are comfortable with an anatomic approach to image interpretation may need to broaden their view. Physiologic imaging of the kind described by Lee et al has the potential to profoundly affect the daily care of the most critically ill patients. Increasing communication between specialists and a collaborative approach to the daily evaluation of critically ill patients will be essential to translate what is technically possible into clinical reality.
Summary
By using a well-established model of acute lung injury, Lee and colleagues (3) demonstrated the equalization of both ventral-dorsal and cephalocaudal ventilation and perfusion gradients in the prone lung, as contrasted with the supine lung, which shows strong gravitationally dependent gradients.
REFERENCES
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